The present invention relates to a color image forming apparatus such as an electrophotographic copier in which: an electrostatic latent image of a document is formed on a photoreceptor (an image forming body); the formed electrostatic latent image is developed into a visual color image (a toner image); and the visual image is transferred onto and fixed on a transfer sheet, using an electrophotographic system.
In the color image forming apparatus using the electrophotographic method or an electrostatic recording method, the electrostatic latent image is formed on the image forming body according to a document image, and the latent image is developed by toners which are charged particles. The above principle is used as follows so that a color image and a synthesized image (superimposition of a plurality of documents or image information on an image of a document) can be obtained. That is, the principle is realized when a cycle of charging, image exposing and developing operations are repeatedly conducted more than two times on the image forming body having a photoconductive layer on a conductive base plate (for example, Japanese Patent Application No. 184381/1983).
In this image forming method, a color developing operation or an image synthesizing operation can be conducted, and the superimposed toner image can be transferred onto the transfer sheet by one transfer process, and therefore, the apparatus, in which the color image or synthesized image can be obtained by a simple structure, can be realized.
As a developing method by which the image forming process is realized, it is necessary that the developing is conducted, for example, using a developer composed of non-magnetic toner and magnetic carrier under the condition disclosed in Japanese Patent Applications No. 57446/1983, and 192712/1985. In a developing device, the following operations are carried out: developer is stirred and toner is, for example, negatively charged; the charged toner adheres to the magnetic carrier surface due to electrostatic force; the developer, in which the toner is electrostatically coupled with the carrier, is magnetically attracted to the surface of a developing sleeve; and the sleeve is rotated at a predetermined linear velocity while the developer is being held on the surface of the sleeve, and conveys the developer to a developing area. Although this developing method is a kind of magnetic brush developing method, the magnetic brush does not contact the image forming body. This method has the following characteristics in which: only the toner is scattered onto a latent image portion according to the document image on the image forming body when a DC bias voltage is supplied or when DC and AC bias voltages are superimposed.
A color image forming apparatus, in which the latent image is formed by a latent image forming means for each color and developed by the developing device in which the color toner corresponding to each latent image is used, is provided as the image forming apparatus to which the foregoing image forming process and developing method are applied.
In the color image forming apparatus, the following apparatus is typically used in which the electrostatic latent image is formed by emitting a light beam, such as a laser beam, on the image forming body (which will be called the photoreceptor, hereinafter) having photoconductive material on the conductive base plate. Alternatively, the laser may also be replaced with an LED, and the emitting fore end of the LED is covered or uncovered with a liquid crystal shutter.
In this type of color image forming apparatus, a color toner image is formed in a toner image forming process shown in FIG. 9(a) to FIG. 9(f).
FIGS. 9(a) to 9(f) are explanatory drawings showing the toner image forming process in which image exposing and developing operations are repeatedly conducted on the photoreceptor according to a document image, and toner images are superimposed on the photoreceptor.
The photoreceptor is uniformly charged when a charger conducts a corona discharge as shown in FIG. 9(a), so that the surface potential of the photoreceptor is uniformly increased up to the surface potential V.sub.H.
A laser exposing device emits a laser beam and forms the electrostatic latent image on the photoreceptor surface. At this time, the surface potential of the photoreceptor surface on which the electrostatic latent image is formed is decreased from the surface potential V.sub.H to the surface potential V.sub.L1 as shown in FIG. 9(b). This surface potential of the photoreceptor V.sub.L1 is that of the electrostatic latent image which was formed initially, and it is defined as an exposure potential V.sub.L1. The surface potential of the electrostatic latent image formed on the photoreceptor surface is determined by the intensity of the laser beam. Thereby, a developing potential gap V.sub.G1 is generated which is a potential difference between the surface potential V.sub.DC of the developing sleeve produced by a DC component of a bias voltage impressed from a developing bias circuit of the developing device, and the exposure potential V.sub.L1. This developing potential gap V.sub.G1 contributes to the next development. Because an electric field generated by this developing potential gap V.sub.G1 is directed from the surface of the latent image on the photoreceptor surface to the surface of the developing sleeve, toner particles, which are negatively charged particles, are attracted by an electric force directed to a latent image portion of the photoreceptor surface. However, the electric force is not so strong as the toner particles, which are electrostatically coupled particles, and are separated from the magnetic carrier. On the other hand, the developer, which is held on the surface of the developing sleeve by magnetic force, is scattered from the developing sleeve to the V.sub.L1 portion of the photoreceptor and adhered to the electrostatic latent image by an electrostatic force, because a large force is further supplied to the developer when the polarity of an AC component of the bias voltage impressed from the developing bias circuit is the same as that of a DC component. Due to the foregoing, the first toner image can be obtained when the latent image on the surface of the photoreceptor, onto which the negatively charged toner particles are electrostatically adhered, is developed (reversal development). At this time, the surface potential of the first toner layer adhered to the electrostatic latent image on the photoreceptor surface is the same as the toner layer surface potential V.sub.T1 as shown in FIG. 9(c).
Next, the photoreceptor surface on which the toner layer has been formed by the foregoing first development is uniformly charged again so that the surface potential of the photoreceptor is the surface potential V.sub.H when the charger conducts a scorotron discharge, and is ready for the next latent image forming process. At this time, a potential distribution of the photoreceptor surface which is approximately uniformly charged is shown in FIG. 9(d), and the surface potential of the photoreceptor is equal to V.sub.H.
The laser exposure device conducts the second image exposure and the second latent image is formed on the surface of the photoreceptor. The potential distribution of the photoreceptor surface at this time is shown in FIG. 9(e). The surface potential of the photoreceptor by the first image exposure is decreased from V.sub.H to the exposure potential V.sub.L1 as described above. The surface potential of the photoreceptor by the second image exposure is decreased from V.sub.H to the re-exposure potential V.sub.L2. The surface potential of the toner layer formed on the surface of the photoreceptor is decreased from the potential which is close to V.sub.H to the potential V.sub.T2. Here, a developing potential gap V.sub.G1, which is a potential difference between the surface potential V.sub.DC of the developing sleeve and the exposure potential V.sub.L1, is generated, and a developing gap V.sub.G2, which is a potential difference between the surface potential V.sub.DC of the developing sleeve and the re-exposure potential V.sub.L2, is generated. Although these developing potential gaps V.sub.G1 and V.sub.G2 contribute to the next development, the electric force generated only by the electric field due to the developing potential gaps V.sub.G1 and V.sub.G2 as described above is not so strong as charged toner particles coupled by the electrostatic force are separated from the magnetic carrier. Also in this case, the force to carry the toner to the photoreceptor is sufficiently large when the polarity of the AC component of the bias voltage impressed from the developing bias circuit is the same as that of the potential gap due to V.sub.G1 and V.sub.G2, so that the toner is scattered and adhered to the photoreceptor. Due to the foregoing, when negatively charged toner particles are electrostatically adhered to the latent image on the photoreceptor surface as shown in FIG. 9(f), and the latent image is developed (reversal development), the second toner layer can be obtained.
A color toner image can be obtained on the photoreceptor when the foregoing processes are repeatedly conducted if necessary. A charging operation, in which the charging polarity is opposite to the toner charging polarity, is conducted by a transfer device from the back of the transfer sheet which is in contact with the toner images, and the toner images are collectively transferred onto the transfer sheet. Further, the transfer sheet is heated and pressurized, so that a color image can be obtained.
In the foregoing transferring operation of the toner image onto the transfer sheet, it is difficult to provide the transfer sheet with a uniform insulation property when humidity becomes high, so that the insulation property of the transfer sheet is partially lowered. Then, transition of electric charges occurs between the transfer sheet and the photoreceptor, and transfer charges are lost. As a result, transfer efficiency is partially lowered, and undesirable nonuniform transfer occurs as if the toner layer were partially torn away. The potential voltage of the color toner image developed by a plurality of color toners on the photoreceptor is approximately equal to that of a non-image portion by means of re-charging except the final color toner image. Accordingly, the potential voltage of a color toner section is irregular, and nonuniformity of transfer easily occurs. A pre-transfer exposure method has been proposed in which the entire surface of the photoreceptor is uniformly charged before transfer in order to prevent the nonuniformity of transfer after all developments have been completed. It was experimentally discovered that the nonuniformity of transfer can be prevented when the pre-transfer exposure process is conducted. However, as shown in FIG. 10, because toner particles are charged to the same polarity, toner particles of end portions of the toner image are scattered around the toner image by repulsive forces, so that the image is stained and image resolution is lowered, which is undesirable. This can be explained as follows. At the time of development, as shown in FIG. 9(c) and FIG. 9(f), the toner image is formed in an electrical potential-well, and therefore, the repulsive force generated by the toner charges is included and left in the potential-well as it is. When electric charges around the toner image are discharged, the potential of the toner image portion extends from the potential-well, and when some toners scatter around the toner image, the repulsive force generated by toner particles is released. Accordingly, the pre-transfer exposure method, by which the entire surface of the photoreceptor is uniformly exposed before transfer, is not a complete solution of the problem.
Accordingly, in order to prevent inferior transfer at the time of high humidity, it is necessary that a heater is provided in the apparatus, and an increase of the humidity of the transfer sheet and the transfer belt is prevented by the heater. Accordingly, the following problems occur: the apparatus becomes complicated; and specially processed and expensive transfer material should be used so that the humidity does not affect the apparatus.
An object of the present invention is to solve the foregoing problems and to provide a color image forming apparatus in which a document image is read out and a color image is formed, wherein the apparatus is not complicated; inferior transfer does not occur even at the time of high humidity; and an image with superior quality can be stably obtained.